Stepped bimetallic shadow mask mounting element

ABSTRACT

In a shadow mask assembly for use in a color picture tube of the type including a frame, a shadow mask supported by the frame, a bimetal element secured to the frame and a leaf spring secured to the bimetal element, the bimetal element comprises an upper side portion secured to the frame, a lower side portion secured to the leaf spring and a flat step intermediate the upper and lower sides, the width of the intermediate flat step gradually varying from one end to the other end of the step.

United States Patent Yamazaki et al.

STEPPED BIMETALLIC SHADOW MASK MOUNTING ELEMENT Inventors: Eiichi Yamazaki, lchihara-shi Chiba-Ken; Takao Kawamura, Mobara-shi, Chiba-ken, both of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Sept. 30, 1971 App]. No.: 185,278

11.8. CI. 313/85 S, 313/292 Int. Cl HOlj 29/02, H01 j 29/06 Field of Search 313/70 C, 69 C, 92 B,

References Cited UNITED STATES PATENTS 8/1970 Rigdon 313/284 X [4 1 Aug. 21, 1973 3,524,974 8/1970 Kautz 313/284 X 3,660,708 5/1972 Nakamura et al. 313/85 S Primary Examiner-Robert Segal AttorneyCraig, Antonelli and Hill In a shadow mask assembly for use in a color picture tube of the type including a frame, a shadow mask supported by the frame, a bimetal element secured to the frame and a leaf spring secured to the bimetal element, the bimetal element comprises an upper side portion secured to the frame, a lower side portion secured to the leaf spring and a flat step intermediate the upper and lower sides, the width of the intermediate flat step gradually varying from one end to the other end of the step.

ABSTRACT 6 Claims, 7 Drawing Figures Patented Aug. 21, 1973.

3 Sheets-Sheet 1 FIG./

PRIOR ART PRIOR ART INVENTORS EHCHI YAMAZA Kl AKAO KAwAMuRA BY awtwam Hum ATTORNEYS Patented Aug. 21, 1973 3,754,157

3 Sheets-Sheet 2 INVENTORS Eu -HI YAMAZAKL TAKAO KAWAMURA BY cm auuuaamam ATTORNEYS Patented Aug. 21, 1973 3,754,157

3 Sheets-Sheet L6 INVENTORS EncHl YAMAZAKI,

TAKAO KAWAMURA, BY w Mm; '6 mm ATTORNEY5 STEPPED BIMETALLIC SHADOW MASK MOUNTING ELEMENT BACKGROUND OF THE INVENTION This invention relates to a color picture tube and more particularly to a shadow mask assembly of a shadow mask type color picture tube.

As shown in FIG. 1 of the accompanying drawing, a conventional shadow mask type color picture tube comprises a fluorescent screen or surface including a mosaic or phosphor dots (not shown) of red, blue and green light emitting phosphors, an electron gun 12 for emitting electron beams for exciting the fluorescent screen 11 and a shadow mask assembly 13 positioned between the electron gun 12 and the florescent screen 11, which are sealed in an envelope of a bulb bulb 14.

Shadow mask assembly 13 comprises a shadow mask 16 made of a thin metal plate provided with a plurality of small perforations over the entire surface, a rectangular frame 17 having an L-shaped cross section and adapted to support the shadow mask, and a plurality of leaf springs 18 for securing the frame 17 to the inner surface of the glass bulb 14. The shadow mask assembly 13 is secured to the inner surface of the glass bulb 14 by the engagement of perforations 19 at the ends of the respective leaf springs with pins 20 projecting from the inner surface of the glass bulb.

Usually, shadow mask 16 is constructed to have substantially the same area as the fluorescent screen 11 and is disposed in close disposition thereto so as to maintain the perforations 15 at a regular relationship with respect to the phosphor dots on the fluorescent screen. Consequently, the electron beams (not shown) emitted from the electron gun 12 are caused to impinge upon corresponding phosphor dots on the fluorescent screen after passing through respective perforations 15 of the shadow mask 16. With the arrangement described above, the electron beams from the electron gun 12 scan the entire surface of the shadow mask so that the electron beams that do not pass through perforations 15 will impinge upon the surface of the shadow mask, thus heating and expanding the same. As the shadow mask is expanded by heat, the above-described regular relationship between the perforations 15 and the phosphor dots is disturbed with the result that the electron beams do not impinge upon corresponding phosphor dots. This phenomenon is remarkable especially at the peripheral portion of the fluorescent screen, thus degrading the purity of the reproduced color and causing a so-called color break-up of the color picture tube.

In the past, various types of shadow mask assemblies have been proposed for the purpose of obviating these difficulties. To have a better understanding of the invention, typical prior art assemblies will first be described with reference to FIGS. 2 to 5. FIGS. 2 and 3 show a prior art assembly utilizing a rectangular stripshaped bimetal element 21 wherein various elements corresponding to those shown in FIG. 1 are designated by the same reference numerals. As shown, one longitudinal edge 21a of bimetal element 21 is disposed substantially in parallel with one edge 17a of frame 17 and is welded thereto; whereas, the opposite side edge 21b is welded to a leaf spring 18 which is disposed substantially in parallel with the longitudinal edge 17a of the frame 17. The portion of the bimetal element 21 having the smaller coefficient of thermal expansion is positioned in contact with the frame 17; whereas, the portion having the larger coefficient of thermal expansion is positioned to face the inner wall of the glass bulb 14. There is a small gap 22 between the longitudinal edge 21b of the bimetal element and the frame 17. The deflection of the bimetal increases in proportion to the distance La between the portion 18a of the spring 18 which is secured to bimetal element 21 and the portion 18b including the opening 19 adapted to receive pin 20 secured to the glass bulb 14. When the shadow mask 16 expands, bimetal element 21 deflects in the direction of the arrow shown in FIG 2 to urge the shadow mask 16 toward the fluorescent screen, thus assuring correct impingement of the electron beams upon corresponding phosphor dots.

With this construction, however, heating of the shadow mask beyond a predetermined temperature causes excessive expansion of the shadow mask, which in turn causes excessive deflection of the bimetal element 21. Such an excessive deflection occurs to such a large extent that it cannot be absorbed by gap 22, thus preventing free flexure of the bimetal element.

FIGS. 4 and 5 illustrate a prior art assembly utilizing a modified bimetal element for the purpose of obviating the problem encountered in the construction shown in FIGS. 2 and 3. The bimetal element 31 shown in FIGS. 4 and 5 is provided with a step 32 of uniform width at about the middle of the width of the strip, the step extending longitudinally substantially parallel with the frame 17. The upper side 31a of the bimetal element is welded to frame 17; whereas, the lower side 31b, which is remote from frame 17, is welded to one end of the leaf spring 18.

With the modified construction illustrated in FIGS. 4 and 5, it is possible to make the air gap 33, defined between the bimetal element 31 and the frame 17, larger than that shown in FIGS. 2 and 3, so that it becomes possible to completely absorb the deflection of the bimetal element 31 caused by the thermal expansion of the shadow mask 16. With this construction, however, the distance Lb between the portion 18a of the leaf spring 18, which is connected to the bimetal element 31, and the portion 18b, including the opening 19 which is adapted to engage pin 20 fixed to the glass bulb 14, becomes smaller than the distance La shown in FIGS. 2 and 3, thus decreasing the effect of the bimetal element.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an improved shadow mask assembly which can completely absorb the heat expansion of the shadow mask caused by the impingement of the electron beams thereon, thus preventing color break-up.

A further object of this invention is to provide an improved shadow mask which can absorb the thermal expansion of the shadow mask by employing a bimetal support element of improved construction.

According to the invention, there is provided a shadow mask assembly for use in a color picture tube of the type including a frame, a shadow mask supported by the frame, a bimetal element secured to the frame and a leaf spring secured to the bimetal element, wherein the bimetal element comprises an upper side portion secured to the frame, a lower side portion secured to the leaf spring and a flat step intermediate the upper and lower side portions, the width of the intermediate step gradually varying from one end to the other end of the step.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will become apparent from the following detailed descrip tion of the invention taken in conjunction with the accompanying drawings in which FIG. 1 is a side elevation, partly in section, of a color picture tube showing the general construction thereof;

FIG. 2 is an enlarged perspective view of a leaf spring adapted to support a shadow mask assembly in a conventional color picture tube;

FIG. 3 is a plan view of the elements shown in FIG.

FIG. 4 is a view similar to FIG. 2 employing a modified bimetal element;

FIG. 5 is a plan view of the elements shown in FIG.

FIG. 6 is a view similar to FIG. 2, but employing an improved bimetal element embodying the principles of the present invention; and

FIG. 7 is a plan view of the elements shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIGS. 6 and 7, in accordance with this invention, the width of the step at the middle of the bimetal element is gradually varied from one end to the other. More particularly, the bimetal element 61 is formed with a step 62 at about its center with upper and lower sides 63 and 64 off-set. The upper side 63 is welded to the side wall of frame 17, while the lower side 64 is welded to one end of a leaf spring 18 of well known construction. The opposite end 18b of the leaf spring is provided with an opening for receiving a pin The invention is characterized by the configuration of the step 62 of the bimetal element. More specifically, the width of the step 62 gradually decreases from one end 62a toward the other end 62b. In other words, the width of the step is gradually decreased toward the end 18b of the spring at which it engages the pin 20. The bimetal element is fabricated by preparing a flat strip, bending it at about right angles along a first bending line to form the upper side 63 to be welded to the frame and a horizontal wall and then bending the horizontal wall again about right angles but in the opposite direction along a second bending line which is not parallel with the first bending line, thus forming the lower side 64 and the flat and tapered step 62. A shadow mask 16 provided with a plurality of perforations for passing the electron beams is secured to the side wall of a shadow mask frame 17 having an L-shaped cross section in a manner well known in the art.

With this construction the bimetal element 61 deflects in proportion to the distance Lc between the bent portion 180 of leaf spring 18 and the portion 18b thereof, which includes opening 19 in the same manner as in the conventional bimetal element. However, it is possible to make the distance Lc substantially larger than the distances La and Lb of the prior constructions shown in FIGS. 3 and 5 because of the tapered construction of the step b2 of the bimetal element. Where the distance 1, between the side wall of frame 17 and the portion 18b of the leaf spring 18 equals 45mm, the

thickness 1 of the bimetal strip 0.76mm, the width 1 of the broader end of the step 62 of the bimetal element 3.8mm, the width 1 of the narrower end of the step 2.3mm, and the length 1 of the step 45.0mm, the distance Lc equals about 4.5mm.

In this manner, according to this invention, as it is possible not only to sufficiently increase the distance Lc between the bent portion 18a of the leaf spring 18 and the end portion 18b thereof which includes opening 19 adapted to receive the mounting pin 20, but also to increase the distance between the side wall of frame 17 and the lower side 64 of the bimetal element, it is possible to increase the deflection of the bimetal thereby to effectively prevent decrease in the color purity of the image due to the thermal expansion of the shadow mask.

It is to be understood that similar sets of the bimetal and the leaf spring are welded to three of four sides of the rectangular frame of the shadow mask assembly incorporated into a color picture tube.

What is claimed is:

l. A shadow mask assembly for use in a color picture tube having a tube envelope including a plurality of pins on an inner surface thereof, said assembly comprising a frame, a shadow mask supported by said frame, support means for supporting said frame including at least one bimetal element having the material of lower coefficient of thermal expansion adjacent said frame including an upper side portion secured to said frame, a lower side portion in a plane different from said upper side portion and a flat step intermediate said upper and lower side portions, the width of said step between said upper and lower side portions gradually varying from one end toward the other end thereof, and a leaf spring having an opening formed in one end portion thereof for receiving one of said pins for mounting said shadow mask, said one end portion of said leaf spring extending from the narrower end of said step away from said bimetal element and being angled toward said inner surface and the other end portion thereof being secured to said lower side portion of said bimetal element.

2. The shadow mask assembly according to claim 1, wherein said bimetal element is formed of a single bimetal strip, said upper side portion being formed by bending said strip at substantially right angles along a first bending line and said lower side portion being formed by bending said strip at substantially right angles but in the opposite direction along a second bending line which is not parallel with said first bending line.

3. The shadow mask assembly according to claim 1, wherein said leaf spring secured to said lower side portion of said bimetal element is bent away from said lower side portion at the narrower end of said step, and said one end portion is angled with respect to the bent away portion of said leaf spring to lie in a plane substantially parallel to the outer periphery of said frame.

4. A shadow mask assembly according to claim 1, wherein said step portion lies within a plane transverse to the planes including said upper side portion and said lower side portion.

5. A shadow mask assembly according to claim 4, wherein said upper side portion and said lower side portion each lie in respective planes which are parallel to the axis of said frame but angled with respect to each other.

6. A shadow mask assembly according to claim 1, wherein said support means includes a plurality of bimetal elements, each secured to a leaf spring connected to said color picture tube. 

1. A shadow mask assembly for use in a color picture tube having a tube envelope including a plurality of pins on an inner surface thereof, said assembly comprising a frame, a shadow mask supported by said frame, support means for supporting said frame including at least one bimetal element having the material of lower coefficient of thermal expansion adjacent said frame including an upper side portion secured to said frame, a lower side portion in a plane different from said upper side portion and a flat step intermediate said upper and lower side portions, the width of said step between said upper and lower side portions gradually varying from one end toward the other end thereof, and a leaf spring having an opening formed in one end portion thereof for receiving one of said pins for mounting said shadow mask, said one end portion of said leaf spring extending from the narrower end of said step away from said bimetal element and being angled toward said inner surface and the other end portion thereof being secured to said lower side portion of said bimetal element.
 2. The shadow mask assembly according to claim 1, wherein said bimetal element is formed of a single bimetal strip, said upper side portion being formed by bending said strip at substantially right angles along a first bending line and said lower side portion being formed by bending said strip at substantially right angles but in the opposite direction along a second bending line which is not parallel with said first bending line.
 3. The shadow mask assembly according to claim 1, wherein said leaf spring secured to said lower side portion of said bimetal element is bent away from said lower side portion at the narrower end of said step, and said one end portion is angled with respect to the bent away portion of said leaf spring to lie in a plane substantially parallel to the outer periphery of said frame.
 4. A shadow mask assembly according to claim 1, wherein said step portion lies within a plane transverse to the planes including said upper side portion and said lower side portion.
 5. A shadow mask assembly according to claim 4, wherein said upper side portion and said lower side portion each lie in respective planes which are parallel to the axis of said frame but angled with respect to each other.
 6. A shadow mask assembly according to claim 1, wherein said support means includes a plurality of bimetal elements, each secured to a leaf spring connected to said color picture tube. 